The host range of and host cell susceptibility to the leukosis-sarcoma virus is dependent on specific interactions between genetically controlled properties of the viral envelope and the host cell plasma membrane. The cell membrane "receptor" may act as an inherited alloantigen and be detectable by existing serological methods. In an effort to produce alloantibodies specific for receptors controlling susceptibility to the viral subgroups A, B, and C, chickens of known receptor genotype and compatible erythrocyte alloantigens at 12 other loci are being cross-transfused. Similar immunizations with bursal cells from selected donors will also be carried out in an effort to produce antibodies against receptors on the surface of lymphocytes. The resulting antisera are being analyzed by using lines of chickens fixed for different combinations of A, B, and C receptors and of known alloantigen constitutions. Another primary objective is to investigate the substructure of the B-histocompatibility complex. The significance of this investigation rests on the finding that certain B haplotypes either resist specific tumor formation or are very effective in regressing them. Currently, several recombinants within the B complex are under investigation for effects of substructure components. Resistance to a malignant lymphoma in the chicken (Marek's disease) is determined by a gene, or genes, within or closely linked to the B-F region of the B complex. Current data show that the B-G region of the major histocompatibility complex of the chicken has been fractionated genetically into two segments. (IB)